In general, an anisotropic conductive connection is necessary to electrically connect connection electrodes of an integrated circuit (IC) board to terminals of a substrate, such as a liquid crystal display (LCD) panel, mounted on a circuit board. Anisotropic conductive packaging materials include the widely-used film-type adhesives, in which conductive particles, such as metal or resin-coated resin particles, are dispersed in an insulating resin, e.g., epoxy, urethane, or acrylic resin. Conductive particles may be interposed between electrodes and terminals by dispersing an anisotropic conductive packaging material containing the conductive particles between the electrodes and the terminals, followed by thermal pressing to adhere the packaging material therebetween. At this time, it is desirable for an electrical connection to occur in the z-axis direction, while an insulating state is maintained in the xy-plane due to the presence of the insulating components in the insulating adhesive. Thus, anisotropic conductivity may be exhibited.
In circuit board packaging requiring anisotropic conductive connections, recent advances in circuit and LCD technologies have brought about a decrease in the size of connection pitches and IC bumps and an increase in the number of leads printed on substrates. Further, there has been a continued need for improved connection reliability. In order to satisfy such technical needs, the conductive particles present in anisotropic conductive films may need to have a small particle diameter. Further, extensive research and development has been continuously conducted to increase the concentration of conductive particles in anisotropic conductive films, in order to improve connection reliability. However, as the diameter of the conductive particles decreases and the density of the particles in the film increases, aggregation of the particles and formation of the bridges between particles may occur, which may create a non-uniform connection and cause frequent shorting between patterns.
Various methods have been proposed to solve the problem of shorting between adjacent electrodes. For example, methods include partially or fully covering the surface of conductive particles with insulating coating materials (e.g., insulating polymer resins) by microencapsulation, spray drying, coacervation, electrostatic coalescence, metathesis polymerization, physical/mechanical hybridization and other processes. See Japanese Patent Laid-Open Nos. Sho 62-40183, Sho 62-176139, Sho 63-237372, Hei 3-46774, Hei 4-174980, Hei 7-105716, 2001-195921 and 2003-313459. Another method described in Japanese Patent Laid-Open No. Hei 2-204917 includes coating the surface of conductive particles with an electrically insulating metal oxide.
Japanese Patent Laid-Open No. Sho 62-40183 describes an anisotropic conductive adhesive film that includes conductive particles that are surface-coated with an insulating resin and then thermally pressed. When the insulating layer is ruptured to expose the conductive layer of the conductive particles, electrical connectivity may be achieved. Japanese Patent Laid-Open No. Sho 63-237372 describes an insulating layer for conductive particles that softens and flows when thermally pressed, which as a result, may allow a portion of the conductive particle to be exposed and electrical connection may be achieved. For both of these methods, however, when the insulating layer is ruptured or softened, respectively, the insulating layer may not be completely removed. Thus, the conductive surface may not be exposed sufficiently to lower the connection resistance. As a result, a stable connection between electrodes may be difficult to achieve, making it difficult to ensure a reliable electrical connection for a sustained period of time. Moreover, the rupturing of the thermosetting layer may cause damage to minute bumps or patterns in the film. In addition, new low-temperature fast curing types of anisotropic conductive films, which have been introduced in order to shorten processing time and reduce the production costs, make it more difficult to rupture or remove the insulating layer, thus further deteriorating connection reliability.
Japanese Patent Laid-Open No. Sho 58-223953, Hei 6-333965, Hei 6-349339 and 2001-164232 describe methods of minimizing the aggregation of particles and improving connection reliability of anisotropic conductive adhesives by further adding insulating organic or inorganic particles, insulating fibrous fillers and the like, in addition to the conductive particles. However, these methods limit the concentration of conductive particles that may be used in the films, and may create further problems, including difficulty in sustaining long-term connection reliability.
Japanese Patent Laid-Open Nos. Hei 3-112-11 and Hei 4-259766 describe methods for producing insulated conductive particles by attaching insulating particles to the surface of the conductive particles. These methods use an additional binder or insulating resin to attach the insulating particles to the surface of the conductive particles. Since the insulating particles are simply bound to the resin physically, the binding force between the insulating particles and the resin is weak. For this reason, when the insulated conductive particles are dispersed into an anisotropic conductive adhesive resin, the insulating particles may aggregate due to added solvents and stirring, and thus, the insulation of the conductive particles may not be sufficient. In addition, despite the separation of the insulating particles under connection conditions (e.g., heating and pressurization), the resin used to attach the insulating particles may not be completely removed, thus deteriorating electrical connectivity and connection reliability.
Various methods have been attempted to prevent the separation of conductive particles from the electrodes by forming anisotropic conductive films in multilayer structures. However, these methods may require extended production time and complicated production procedures, which are undesirable from a processing standpoint. These methods are therefore generally less desirable than methods using insulated conductive particles.
It would therefore be desirable to provide insulated conductive particles with superior electrical connectivity in the pressurizing direction and with high connection reliability. It would further be desirable to provide insulated conductive particles that can eliminate or reduce shorting between adjacent bumps or interconnection patterns by preventing aggregation of conductive particles. It would further be desirable to provide insulated conductive particles having superior solvent resistance in adhesive resin compositions. It would further be desirable to provide insulated conductive particles that may be used in compact interconnection pattern pitches and low-temperature fast-curing anisotropic adhesive films.